The ebuild file of zlib is processing only to .so file. However, not .so file but .dylib is created in macos. so, zlib fails to emerge on macos. This patch solves the problem.
Created attachment 35668 [details, diff] patch for zlib-1.2.1-r2 ebuild
I'd like to apply the patch but the ebuild overwrite existing system-zlib. I suggest that we add this patch after we switched to use Portage specific prefix (to not overwrite zlib default installation).